1. Field of the Invention
The present invention relates to a stainless steel pipe and a joining method thereof adapted for use in a semiconductor manufacturing apparatus. A pipe or the like for supplying a gas thereto, where mixing of impurities made of oxides or the like generated by contacts with an inner wall of a pipe, must be extremely prohibited.
2. Description of the Prior Art
Since a gas used in a semiconductor manufacturing apparatus or a pipe for supplying a material gas thereto has a strong corrosiveness, a stainless steel having a high corrosion resistance has been frequently used in an apparatus or a pipe, contacting the gas. TIG welding (Tungsten Inert-Gas arc welding) has been generally used in such welding for joining where stainless steel pipes are connectively arranged at a predetermined length.
In semiconductor manufacturing facilities or pipes annexed thereto for providing a gas thereto, it is preferable there exists as much as possible no unnecessary space referred to as xe2x80x9cdead spacexe2x80x9d in a channel pipe with the gas passing therethrough. That is to say, if there exists a dead space in the pipe, dust or particles accumulate in the dead space such that the particles are detached therefrom due to some shock thereby being contained within a passing gas and then discharged therewith.
If there exists a dead space in such a pipe when a corrosive gas is supplied therethrough, it is difficult to remove water existing in the dead space and a corrosive gas is dissolved in the water to thereby facilitate arousing a crevice corrosion. If crevice corrosion occurs, metallic corrosive products come to be exfoliated from an inner wall and become impurities to thereby contaminate the gas.
A dead space is frequently generated during installation of piping or manufacturing facilities, especially during a welding process for connection. For example, in case a welded portion for connecting of stainless steel pipes lacks of penetration, there remains an unmelted butt joining portion in an inner wall surface portion of a welded portion of a pipe so that a microscopic cavity thereof becomes dead space. It has been thus required to carry out a welding with complete penetration such that welded metal for welding reaches an inner wall surface of a welded portion.
However, it has been recently known that if welded metal reaches an inner wall surface of a welded portion of a pipe, ultramicroscopic metallic particles referred to as fume are generated from the welded metal toward the inside of a pipe so that the particles become a cause to contaminate a passing gas.
Even though it has been required to use a stainless steel pipe of a low impurity content and a high price due to the above-mentioned problem, there still remains the problem of generation of fume.
Moreover, since there exist welded metal and a heat-affected zone, of a low corrosion resistance in a welded portion with complete penetration, there remains another problem of generating corrosion in that region.
The present invention is provided to solve the above-mentioned problems and it is an object of the present invention to provide a stainless steel pipe which prevents dead space from being generated on an inner wall surface portion of a joining portion thereof and prevents gas from being contaminated by fume of metallic particles generated from a welded portion of a pipe such that passing gas to be supplied is not contaminated by impurities. It is still another object of the present invention to provide a stainless steel pipe having excellent corrosion resistance in a joining portion. It is still another object of the present invention to provide a joining method for obtaining the above-mentioned pipes.
According to the present invention, there is provided a stainless steel pipe joined by butt welding, the stainless steel pipe comprising: a joining portion having a butt joining portion; the butt joining portion having an outer butt joining portion in an outer wall surface portion joined with penetration; and, an inner butt joining portion in an inner wall surface portion joined in solid-state without being melted such that the joining portion comprises the butt joining portion joined by the butt welding with partial penetration.
Moreover, there is also provided a stainless steel pipe as defined above, wherein a dimension ts of the unmelted inner wall surface portion in solid-state is set within the range of 0 less than tsxe2x89xa60.7(mm) starting from the inner wall surface as a reference surface toward the outer wall surface with regard to a thickness t when the thickness t of the stainless steel pipe is more than 1 mm.
Moreover, there is provided a stainless steel pipe as defined above, wherein the dimension ts of the unmelted inner wall surface portion in solid-state is set within the range of 0 less than ts less than 0.7(mm) starting from the inner wall surface as a reference surface toward the outer wall surface with regard to a thickness t when the thickness t of the stainless steel pipe is less than 1 mm.
Moreover, there is provided a joining method for joining a stainless steel pipe comprising the steps of attaching butt joining portions of stainless steel pipes to be joined; welding the butt joining portions under back shield gas atmosphere or vacuum surrounding such that inner wall surface portions of the butt joining portion are joined in solid-state without being melted and outer wall surface portions of the butt joining portion are joined with penetration.
Moreover, there is provided a joining method as defined above, wherein a content of an oxidative gas is less than 10 ppm under a back shielding gas atmosphere or vacuum surrounding.